Lisa, your local county animal control will come and pick up animals like this. Like you, when ever I see an injured animal it stays with me for days.
I once had a female long haired calico cat show up in my backyard. She was old, starving and had no teeth. So I fed her some wet food and then started working on her coat. It took me 5 hours with a razor blade to cut away the mats of hair literally so twisted her skin was raw from it. She purred and purred while I worked away. I went through 10 razor blades. Under each huge mat were colonies of fleas. I was crying my eyes out as I worked. After each chunk was released she went into a frenzy of itching, and then would come back for more.
My neighbor is a vet tech, so I called her over to give me the prognosis. She said it was obvious she was almost blind from Diabetes and she needed to be put down. The only comfort I had was that her last day on earth, someone had cared about her, but that was for a tame animal not a wild one.
Gregor....now I can't get your story out of my mind. All we can do is our best. No one should feel bad about not knowing what to do or who to call.
r.